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Investments - Investment in joint ventures (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended
	Oct. 31, 2014	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Investments
Investment in WMES joint ventures at beginning of the period				$ 41,590
Capital contribution				630
Earnings from joint ventures		$ 215	$ 245	569	$ 551
Investment in WMES joint ventures at end of the period		42,789		42,789
WMES
Investments
Investment in WMES joint ventures at beginning of the period				26,672
Capital contribution				630
Earnings from joint ventures				644
Investment in WMES joint ventures at end of the period		27,946		27,946
CASC
Investments
Investment in WMES joint ventures at beginning of the period				14,918
Capital contribution	$ (15,000)
Earnings from joint ventures				(75)
Investment in WMES joint ventures at end of the period		$ 14,843		$ 14,843